I putting together a new system and have a Corsair RM550x power supply and a ASRock Extreme4 x390 motherboard. For the CPU it has an 8pin and a 4 pin connector but on the powersupply I have one have 8 pin connector. I also have 6 pin connector that can split to offer 4 pin as well but that will be used for the graphics card, GTX 1080.

I've read that using the additional 4 pin is not required and indeed the system does POST to BIOS with just the 8 PIN how ever the manual states the additional 4 pin is required.

Looking around seems most X390 boards have the 8+4pin except for the cheaper boards with less phases.

So is the RM550x just not compatible with most Z390 board or is there another cable I can get that will allow this to work?

I've seen there is peripheral power (molex) to 4 pin adapters but not sure about those and if that is the case is there a cable I can get that will fit into the back of the power supply peripheral section that goes straight to 4 pin?